1. Field of the Invention
The present invention relates generally to an evaporative emission control system for automotive vehicles and, more particularly, to a leak detection assembly for determining if a leak is present in a portion of the system which includes a vapor collection canister.
2. Description of the Related Art
Modern automotive vehicles typically include a fuel tank and an evaporative emission control system that collects volatile fuel vapors generated in the fuel tank. The control system includes a vapor collection canister, usually containing an activated charcoal mixture, to collect and store the emitted fuel vapors. Normally, the canister collects volatile fuel vapors which accumulate during refueling of the automotive vehicle or from increases in fuel temperature. During conditions conducive to purging, a purge valve placed between an intake manifold and the canister is opened by an engine control unit in an amount determined by the engine control unit to purge the canister; i.e., the stored vapors are drawn into the intake manifold from the canister for ultimate combustion within a combustion chamber of an engine.
Governmental regulations require that certain vehicles powered by volatile fuels such as gasoline have their evaporative emission control systems checked to determine if a leak exists in the system. On board vehicle diagnostic systems have been developed to determine if a leak is present in a portion of the evaporative emission control system. One such system utilizes a vacuum regulator/sensor unit to draw a vacuum on the control system and sense whether a loss of vacuum occurs within a specified period of time.
Diagnostic systems also exist for determining the presence of a leak in the evaporative emission control system which utilize positive pressurization rather than negative pressurization, i.e. a vacuum. In positive pressurization systems, the evaporative emission control system is pressurized to a set pressure, typically through the use of an electric air pump. A sensor determines whether the pressure remains constant over a certain amount of time.
Positive pressurization systems have a benefit over negative pressurization systems in that the increased pressure suppresses the rate of fuel vapor generation in the fuel tank. Such a situation is desirable when the test is given under hot weather conditions which typically promote fuel vapor generation.